This invention relates to a method of decomposing hydrogen peroxide in aqueous solution, more particularly, to a method that is capable of effective and economical decomposition of hydrogen peroxide in waste acids or wastewaters.
Hydrogen peroxide is a strong oxidizer and used in various applications including disinfection and deodorizing of containers of medicines or vessels or tools in the fermentation industry, purification of fats and oils, bleaching of textile fibers, bleaching and sterilization of processed marine products, as well as the manufacture of drugs. Hydrogen peroxide is also used in rocket fuels either independently or in admixture with kerosine or the like.
The use of hydrogen peroxide has recently expanded to the electronics industry, where it is added to sulfuric acid and used for cleaning silicon substrates and other operations in the process of IC and LSI fabrication. Further, the spread of waste paper recycling activities contributed to the recent expansion of the demand of hydrogen peroxide as deinking and bleaching agent or for use in the treatment of plant effluents, as evidenced by the two-digit annual increase of its use in the three to four years that preceded 1990.
Among the various uses of hydrogen peroxide listed above, its use in processed marine products is regulated by the Food Sanitation Act and related notifications, which require that hydrogen peroxide be decomposed or removed before the final food is produced. As a result of this treatment for removal, hydrogen peroxide can evolve and remain in the wastewater but there are no particular regulations concerning the removal of such residual hydrogen peroxide. However, irrespective of variation in the uses of hydrogen peroxide, the residual hydrogen peroxide will eventually get into the wastewater or other liquid wastes and it must be decomposed completely before those wastes are discharged. Unfortunately, except for catalytic decomposition using metals such as platinum and palladium or oxides of metals such as manganese, cobalt, copper and silver, there are no existing methods that are effective and appropriate for decomposing the residual hydrogen peroxide. Particularly, there are no methods that ensure satisfactory results in the process of decomposing a small or tiny amount of the residual hydrogen peroxide. To confirm this point, the present inventors searched through patent specifications that were prepublished (laid open to public inspection) in the past ten-odd years and found the following patents that are apparently relevant and worth review: Japanese Patent Public Disclosure No. 186208/1986, "Method of decomposing hydrogen peroxide", filed by Shoko Tsusho K.K., and Japanese Patent Public Disclosure No.83287/1982, "Method of eliminating hydrogen peroxide", filed by Kyowa Hakko Kogyo Co., Ltd.
However, the method of the first patent is an improved catalytic method and judging from the examples, it would be difficult to decompose hydrogen peroxide completely (to the 1 ppm level). In addition, handling hydrogen gas is not only cumbersome but also dangerous in practice. Furthermore, the patent states explicitly that the catalysts it uses suffer from the problem of decrease in activity.
The second patent, Japanese Patent Public Disclosure No. 83287/1982, provides an alternative to catalase for use in analytic methods and it states that the objective of the claimed invention is to accomplish quantitative determination of a sample. Considering the possibility that the catalase substitute may remain in solution after decomposition, the method of the second patent will find only limited use.
Papermaking mills currently use chlorine-containing compounds such as sodium hypochlorite to bleach pulp. However, it has recently been pointed out that toxic dioxins can occur during the incineration of wastes and a shift to oxygen-containing bleaching agents such as hydrogen peroxide and ozone is under review.
The use of hydrogen peroxide as a bleaching agent involves one serious problem; that is, residual hydrogen peroxide will eventually get into the wastewater but there is no effective and economical method that can be used to decompose such residual hydrogen peroxide in the wastewater. The method described in Japanese Patent Public Disclosure No.186208/1986, supra, is intended to prevent the drop in catalyst activity and in order to enhance the percent decomposition of hydrogen peroxide in wastewaters, hydrogen gas must be supplied to the catalyst in column in a considerably larger amount than the wastewater to be passed through the column.
As already mentioned, sulfuric acid supplemented with hydrogen peroxide is used for cleaning silicon wafers and for other purposes in the process of IC and LSI fabrication. The problem with this practice is that it gives rise to a waste acid containing hydrogen peroxide; this waste acid is difficult to handle because if it is simply discharged into rivers and other water bodies after neutralization, it can cause the problem of pollution with hydrogen peroxide.
It has been proposed that the sulfuric acid content of the waste acid be recycled for effective use as a substitute neutralizer for pH adjustment in the treatment of wastewaters. However, this approach also involves difficulty due to the presence of harmful hydrogen peroxide in wastewaters. Substitution for the sulfuric acid that is used in the manufacture of chemicals is also difficult to implement since it involves rapid decomposition of hydrogen peroxide or unwanted side reactions such as oxidation.